Voltage Regulators

1. Voltage Regulators • Used to regulate input voltage from a power source • Maintains power level to within set tolerance • Prevents damage to components by acting as a buffer • Two types: Linear and Switching

2. Linear Regulators • Acts like a voltage divider • Uses FET in ohmic region

3. Switching Regulator • Switches on and off rapidly to alter output • Requires Control Oscillator and charge storage components

4. Types of Switching Regulators • Buck (step down) - lowers input • Boost (step up) - raises input • Buck / Boost - lowers/raises/inverts input depending on needs and controller • Charge Pump – provides multiples of input without using any inductors

5. Linear Advantages • Simple     • Low output ripple voltage     • Excellent line and load regulation     • Fast response time to load or line changes     • Low electromagnetic interference (less noise)

6. Linear Disadvantages • Low efficiency     • Large space requirement if heatsink is needed  • Can not increase voltage above the input

7. Switching Advantages • High Efficiency     • Capable of handling higher power densities     • Structures can provide output that is greater than, less than or spanning the input voltage 

8. Switching Disadvantages • Higher output ripple voltage     • Slower transient recovery time     • EMI is produced  (Very noisy) • Generally more costly

9. Linear RegulatorsDarlington NPN Regulator • Dropout= 2Vbe + Vsat; Typically 1.5-2.5Vdc

10. PNP LDO Regulator • Dropout= Vsat; Typically <500mV. • At light loads, this falls to 10-20mV.

11. Quasi-LDO Regulator • Dropout= Vbe + Vsat.

12. Linear Regulators • Two Considerations Ground Pin Current (Ignd), Vdo (Dropout Voltage At light loads, this falls to 10-20mV. • Here, Ignd= IL/β (Pass Transistor Gain) • Darlington’s high gain allow Ignd= a few mA (typically) • Quasi-LDO, good performance (source 3A @ <10mA) • LDO, Ignd= 10-20mA, which can be up to 7% of IL • All are unconditionally stable (requires no external capacitors)

13. P-FET LDO Regulator • Advantageous because the amount of PWR dissipated is Vin * Ignd, because of the Pass FET’s low “on” voltage (~.7-.8V), only a small current is required to maintain regulation.

14. Switching Regulators – Overview • Operation relies on controlled transfer of charge from input to output. • Output node charges while switch is closed and discharges while switch is open. • Requires multi-part circuitry. • Storage of the charge to be transferred. • Control of switching scheme. • Output-stage filtering

15. Switching Regulators – Pulse Rate Modulation • Constant duty cycle • Varying frequency • Noise spectrum imposed by PRM varies and is more difficult to filter out.

16. Switching Regulators – Pulse Width Modulation • Constant frequency • Varying duty cycle • Preferred – Efficient and easy to filter out noise.

17. Switching Regulator – Continuous Mode • Current through inductor never drops to zero. • Allows for highest output power for a given input voltage, switching scheme and switch current rating. • Overall performance is better.

18. Switching Regulator – Discontinuous Mode • Current through inductor drops to zero during each cycle. • Allows for smaller inductor and thus smaller overall circuit size. • Better when output current is low.

19. Switching Topologies Dielectric isolation vs. non-isolating • NI: Uses – small change in Vout/Vin • DI: Uses – radiation-intense environments • Ideally Power in = Power out

20. Non-Isolating forms • Examples include: Buck, Boost, Buck/Boost, Cuk, Charge Pump

21. Buck, Boost, and Buck/BoostCircuits

22. Isolated types • Examples include: Flyback converters and Forward Converters

23. Other Considerations for Switchers • MOSFETs and Diodes • Synchronous Rectification – Getting more Efficiency • Operating Frequencies

24. Ideal Inductors Purely Inductive

25. Real Inductors SRF Inductive Capacitive Resistive

26. Frequency-Dependence for Switching Regulators • The frequency-dependent effects limit the inductor to a useful range of frequencies. • Usually the SRF (Self-Resonant Frequency) is specified by the manufacturer. • This sets a hard upper limit. The useful limit is generally lower.

27. High Frequencies are Good • As a general rule of thumb, doubling the switching frequency allows halving the inductance. • An old DC-DC converter at 30kHz might need an inductor measuring 1” in diameter. • A modern converter operating at 1MHz can deliver the same current with an inductor measuring less than .25” in diameter. • A smaller inductor will have a smaller series resistance.

28. Series Resistance • Series resistance increases losses. • It can also lead to instability in boost configurations. Duty Cycle Duty Cycle

29. Series Resistance - Solutions • Current sensing / overcurrent protection • Duty cycle limiting Duty Cycle

30. Cores and Saturation • Magnetic cores increase the magnetic flux density when an external field is applied, increasing energy storage capacity and inductance. • When a core is saturated, increasing the external magnetic field results in a negligible increase in flux density. 1μm No External Field In an External Field

31. Cores and Saturation • The onset of saturation can occur very rapidly. • At saturation, the effective inductance decreases dramatically. • This decrease can lead to instability (increasing duty cycle, decreasing inductance). • Without current or duty cycle limiting, this can result in catastrophic failure. • The manufacturer will usually specify this as the DC saturation current, the point where the inductance reaches 10% of nominal.

32. Cores and Saturation B • Cores also exhibit losses due to internal “friction” and eddy currents. • In switchers, these losses are usually only a small percentage of the total losses, even at saturation. • However, saturation in a DC-DC supply can cause greater resistive losses due to the drop in inductance. H

33. Important Parameters • Inductance: Larger inductances have lower ripple current, but are larger and can decrease stability. • Peak Current: This is obtained from the switcher’s datasheet. • DC Current: Dependent on load requirements. • SRF: Should be well above the switcher’s operating frequency. • Series Resistance: Affects the efficiency and can affect stability.

34. A Variety of Inductors

35. Applications and Sample Circuits of Some Common Packages • National Instruments LM337 Package • 3-terminal adjustable regulator • Capable of supplying 1.2V to 37V with 1.5A • Offers overload protection that is only available in ICs • Can also be used as a precision current regulator • Texas Instruments TPS32xx Package • Optimal for battery powered portable applications. • Operates a 93% efficiency at 3MHz.

36. Simple LM117 Voltage Regulator Configuration • 1.2V to 25V output • C1 is only needed if the device is more that 6” from the filter capacitors. • C2 is optional – it is used to improve the transient response.

37. 5V Logic Regulator with Electronic Shutdown • Uses TTL for electronic shutdown. • Clamping of adjustment terminal programs the output to 1.2V where most loads draw little current. • 1.2V is the minimum output voltage.

38. 0V to 30V Regulator • The downside is that the full output current is not available for high input/output voltages.

39. Precision Current Limiter • Iout = Vref/R1 • 0.8 Ohms < R1 < 120 Ohms

40. AC Voltage Regulator • Regulates a 24Vp-p Input to 12Vp-p at 1A.

41. 12V Battery Charger • Rs is used to set the output impedance of the charger:

42. Texas Instruments TPS62300 • Used in low-power portable electronics • Designed for fast start-up time

43. References • TI. Understanding Buck Power Stages in Switchmode Power Supplies. http://focus.ti.com/lit/an/slva057/slva057.pdf. • TI. Understanding Boost Power Stages in Switchmode Power Supplies. http://focus.ti.com/lit/an/slva061/slva061.pdf. • Erickson, R. DC-DC Power Converters. http://ece-www.colorado.edu/~rwe/papers/Encyc.pdf. • Shen, L., Kong, J. Applied Electromagnetism, Third Edition. • http://www.web-ee.com/primers/files/f5.pdf • http://www.maxim-ic.com/appnotes.cfm/appnote_number/710/ln/en • http://www.eetasia.com/ARTICLES/2000NOV/2000NOV30_AMD_AN2.PDF